Chicken Pox Cases Have Dropped 85% Since Implementation of 2-Dose Vaccine

• New data from the Centers for Disease Control and Prevention show urgency of vaccination adherence.
• Between the period 2005–2006—before the 2-dose vaccine recommendation—and 2013–2014, the overall incidence of varicella infection declined by 84.6%.
• The largest declines were reported among children aged 5 to 9 years (89.3%) and 10 to 14 years (84.8%).

The incidence of varicella infection (Figures 1 and 2) in children (Figure 3 and 4) has decreased by about 85% since the implementation of the 2-dose vaccine in 2006, according to the Centers for Disease Control and Prevention (CDC). The results were reported recently in Morbidity and Mortality Weekly Report (MMWR).

The CDC currently calls for children to receive the first dose of varicella vaccine at age 12 to 15 months and the second dose when they are aged 4 to 6 years.

Between the period 2005 to 2006—before the 2-dose vaccine recommendation—and 2013 to 2014, the overall incidence of varicella infection declined 84.6%. The largest reported reductions were seen in children aged 5 to 9 years (89.3%) and 10 to 14 years (84.8%).

These data underscore the necessity of adherence to the CDC’s 2-dose vaccination regimen in combating this important childhood disease.

"Further reduction in the number of varicella cases will provide states with increased opportunities for enhancing varicella surveillance and improving completeness of reporting to monitor impact of the vaccination program," wrote the corresponding author Adriana Lopez, MHS, of the Division of Viral Diseases at the National Center for Immunization and Respiratory Diseases.

‘Mild disease occurred significantly more frequently among vaccinated patients than among unvaccinated patients.’

—Morbidity and Mortality Weekly Report

The Data

During the period between 2005 and 2014, the number of states reporting data on the numbers varicella infections to the CDC increased from 27 in 2005 to 40 in 2014. “The average annual varicella incidence declined significantly (84.6%) from 25.4 per 100,000 population during 2005 to 2006 to 3.9 per 100,000 population during 2013 to 2014 (P<0.001),” the CDC noted in MMWR. “Statistically significant declines in incidence were reported for all age groups during this time, with the largest declines among children aged 5 to 9 years (89.3%) and 10 to 14 years (84.8%).”

“In the four states (Illinois, Michigan, Texas, and West Virginia) that have been reporting varicella cases annually since before implementation of the varicella vaccination program, incidence declined an average of 97.4% from 1993 to 1995 and from 2013 to 2014 (range=92.9%–97.9%),” the MMWR authors added.

Data on the vaccination status of patients infected with varicella was available for 12,784 (59.8%) cases. “Of those cases, 7,000 (54.8%) occurred in people who had received ≥1 dose of varicella vaccine. Among these reports, the number of doses received was reported for 2,266 (32.4%) patients, including 921 (39.0%) persons who had received 1 dose of varicella vaccine, 1,331 (56.4%) who had received 2 doses, and 14 (0.6%) who were reported to have received 3 doses,” noted MMWR.

Among the cases reported to the CDC, 3,715 (17.4%) included information about hospitalization. “Among these reports, 81 (2.2%) indicated that the patient was hospitalized. Reports from 17 (22.4%) of 76 hospitalized patients with information on vaccination status indicated receipt of varicella vaccine, and 13 vaccinated hospitalized patients had information on number of doses. Eight patients had received 1 dose, and 5 had received 2 doses,” explained the MMWR authors.

“Among the 8,358 (39.1%) case reports with data on the number of skin lesions, 4,269 (51.1%) were considered to have had mild disease and 4,089 (48.9%) had moderate-to-severe disease. Mild disease occurred significantly more frequently among vaccinated patients (76.8%) than among unvaccinated patients (23.2%) (P<0.001),” they explained.

The link between specific cases of varicella infection and larger outbreaks of the disease was available for 13,826 (64.6%) reports. Of these, 2,279 (16.5%) were associated with an outbreak.

Laboratory confirmation data were available for 2,240 (24.6%) of 9,104 reported cases. Among these cases, 1,842 (82.2%) tested positive by either polymerase chain reaction, direct fluorescent antibody testing, immunoglobulin M by enzyme-linked immunosorbent assay, or viral culture.